There was a long road between the release of IV and the release of VI, and iirc, they took their dear sweet time actually announcing that VI was leaving Japan. For at least a little while it had been starting to feel like VI was going to miss the Western world once more, which wouldn't have been that surprising depending on how SE felt about the performance of IV and V on the DS.
Dragon Quest 4 was supposed to be localized as early as the PS1 version, but its developer went bankrupt as Enix found themselves with an English script they couldn't fit in the PS1 game. So they made it a point to have the engine for the DS remake(s) support English from the get-go.
Japanese version of DQ4 has only the prologue and few other texts translated (for all Multi-5 languages) and the party chat was already removed, so it was more like a proof-of-concept localization (kind of like the Nintendo-made Card Battler DS Japan-only game having some bits and pieces from the intro having English subtitles).
Dragon Quest 6 DS was actually cancelled. Square had Alpha Plus localize parts of it but then killed the project. Nintendo stepped in after a while and hired 8-4 to finish the remaining stuff for 6 and Joker 2 and then published both themselves.
Then, for some idiotic reason, even knowing these circumstances, people boycotted Joker 2 deluding themselves into thinking Nintendo would see the results and then localize the "Plus" re-release. Result was Nintendo not picking up any of the 3DS games, not even the Slime MoriMori one. Oh, and "Softboiled Hero"/"Nanashi No Game" were also caught in the aftermath - with almost finished English versions getting cancelled too because they "won't sell".
As if people just don't learn. Nintendo listened to fans in 2005 and made the gamble of localizing a bunch of obscure Japan-only JRPGs (Magical Starsign, and Tales of Phantasia). People just go -oh that Office 2003 autocorrect typo is evidence the translation is shit, boycott this! both bombed horribly due to terrible word of mouth (ToP's translation was pretty alright), and as a result, none of the Tales DS titles or Mother 3 gets released over here.